Semiconductor chip manufacturing is a good example of the use of equipment for "multi-task" processing. The term "chip" refers to a piece of semiconductor material scribed or etched from a semiconductor wafer on which one or more electronic components are formed. The process by which a wafer is transformed into a chip can be conceptualized as a series of discrete process steps, which transform a bare silicon wafer at the first process step to a finished integrated circuit at the last process step. The "frontend" of the fabrication process results in a semiconductor wafer having fully formed integrated circuits thereon. The individual circuits are separated and packaged elsewhere.
These front-end steps are independent of the machines physically located on the factory floor, and are the functional description of what happens to the wafers at each stage of manufacture. Ideally, processing steps should be scheduled in a manner that most efficiently uses the equipment.
A complicating factor in scheduling is that the process steps do not occur as a single "assembly line" string of events. It is sometimes necessary for a single piece of equipment to operate more than once on a wafer. For example, during the fabrication of a single wafer, the diffusion of impurities into selected openings, or windows, in the semiconductor is repeated many times. The process of diffusing these "windows" involves at least four basic steps: mask generation, oxidation, photolithography and etching, and diffusion. For devices such as transistors and diodes, the diffusion is repeated for different layers, such as for the base, emitter, and collector, or for the anode and cathode. Thus, for example, a photolithography machine might be re-used at different layers to produce a single chip. At any one time, the queue of jobs waiting at that machine could include wafers at various stages of manufacture.
Another complicating factor is that multiple process flows may operate in one facility at the same time, such as when a front-end facility has several product lines. Each product line will have different steps for the manufacturing process. A single machine may perform process steps for each product line flow.
Existing scheduling techniques are not designed for such multi-task equipment. They do not perform well for situations in which the same equipment is used multiple times for a single lot. A need exists for an improved scheduling method for multi-task equipment.